Jewish Calendar

Level: Basic

 Based on moon cycles instead of sun cycles
 "Leap months" are added to sync up with sun cycles
 Used to be calculated by observation
 Calculated mathematically since 4th century
 Years are numbered from Creation

A few years ago, I was in a synagogue, and I
overheard one man ask another, "When is
Chanukkah this year?" The other man smiled
slyly and replied, "Same as always: the 25th of Kislev." This humorous comment
makes an important point: the date of Jewish
holidays does not change from year to year. Holidays are celebrated on the
same day of the Jewish calendar every year, but the Jewish year is not the same
length as a solar year on the civil calendar used by most of the western world,
so the date shifts on the civil calendar.

Background and History

The Jewish calendar is based on three astronomical phenomena: the rotation of
the Earth about its axis (a day); the revolution of the moon about the Earth (a
month); and the revolution of the Earth about the sun (a year). These three
phenomena are independent of each other, so there is no direct correlation
between them. On average, the moon revolves around the Earth in about 29½
days. The Earth revolves around the sun in about 365¼ days, that is, about
12.4 lunar months.

The civil calendar used by most of the world has abandoned any correlation
between the moon cycles and the month, arbitrarily setting the length of months
to 28, 30 or 31 days.

The Jewish calendar, however, coordinates all three of these astronomical
phenomena. Months are either 29 or 30 days, corresponding to the 29½-day
lunar cycle. Years are either 12 or 13 months, corresponding to the 12.4 month
solar cycle.

The lunar month on the Jewish calendar begins when the first sliver of moon
becomes visible after the dark of the moon. In ancient times, the new months
used to be determined by observation. When people observed the new moon, they
would notify the Sanhedrin. When the Sanhedrin heard testimony from two
independent, reliable eyewitnesses that the new moon occurred on a certain
date, they would declare the rosh chodesh (first
of the month) and send out messengers to tell people when the month began.

The problem with strictly lunar calendars is that there are approximately 12.4
lunar months in every solar year, so a 12-month lunar calendar is about 11 days
shorter than a solar year and a 13-month lunar is about 19 longer than a solar
year. The months drift around the seasons on such a calendar: on a 12-month
lunar calendar, the month of Nissan, which is supposed to occur in the Spring,
would occur 11 days earlier in the season each year, eventually occurring in
the Winter, the Fall, the Summer, and then the Spring again. On a 13-month
lunar calendar, the same thing would happen in the other direction, and faster.

To compensate for this drift, the Jewish calendar uses a 12-month lunar
calendar with an extra month occasionally added. The month of Nissan occurs 11
days earlier each year for two or three years, and then jumps forward 30 days,
balancing out the drift. In ancient times, this month was added by observation:
the Sanhedrin observed the conditions of the weather, the crops and the
livestock, and if these were not sufficiently advanced to be considered
"spring," then the Sanhedrin inserted an additional month into the calendar to
make sure that Pesach (Passover) would occur in the spring (it is, after all,
referred to in the Torah as Chag he-Aviv, the Festival of Spring!).

A year with 13 months is referred to in Hebrew as Shanah Me'uberet (pronounced
shah-NAH meh-oo-BEH-reht), literally: a pregnant year. In English, we commonly
call it a leap year. The additional month is known as Adar I, Adar Rishon
(first Adar) or Adar Alef (the Hebrew letter
Alef being the numeral "1" in Hebrew). The extra month is inserted before the
regular month of Adar (known in such years as Adar II, Adar Sheini or Adar
Beit). Note that Adar II is the "real" Adar, the one in which
Purim is celebrated, the one in which
yahrzeits for Adar are observed, the one in
which a 13-year-old born in Adar becomes a Bar
Mitzvah. Adar I is the "extra" Adar.

In the fourth century, Hillel II established a fixed calendar based on
mathematical and astronomical calculations. This calendar, still in use,
standardized the length of months and the addition of months over the course of
a 19 year cycle, so that the lunar calendar realigns with the solar years. Adar
I is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle.
The current cycle began in Jewish year 5758 (the year that began October 2,
1997). If you are musically inclined, you may find it helpful to remember this
pattern of leap years by reference to the major scale: for each whole step
there are two regular years and a leap year; for each half-step there is one
regular year and a leap year. This is easier to understand when you examine the
keyboard illustration below and see how it relates to the leap years above.

In addition, Yom Kippur should not fall
adjacent to Shabbat, because this would cause
difficulties in coordinating the fast with Shabbat, and
Hoshanah Rabbah should not fall on Saturday
because it would interfere with the holiday's observances. A day is added to
the month of Cheshvan or subtracted from the month of Kislev of the previous
year to prevent these things from happening. This process is sometimes referred
to as "fixing" Rosh Hashanah. If you are interested in the details of how these
calculations are performed, see The Jewish Calendar: A
Closer Look.

Numbering of Jewish Years

The year number on the Jewish calendar represents the number of years since
creation, calculated by adding up the ages of people in the Bible back to the
time of creation. However, this does not necessarily mean that the universe has
existed for only 5700 years as we understand years. Many
Orthodox Jews will readily acknowledge that the
first six "days" of creation are not necessarily 24-hour days (indeed, a
24-hour day would be meaningless until the creation of the sun on the fourth
"day"). For a fascinating (albeit somewhat defensive) article by a nuclear
physicist showing how Einstein's Theory of Relativity sheds light on the
correspondence between the Torah's age of the
universe and the age ascertained by science, see
The Age of the Universe.

Jews do not generally use the words "A.D." and "B.C." to refer to the years on
the civil calendar. "A.D." means "the year of our L-rd," and we do not believe
Jesus is the L-rd. Instead, we use the abbreviations C.E. (Common or Christian
Era) and B.C.E. (Before the Common Era), which are commonly used by scholars
today.

Months of the Jewish Year

The "first month" of the Jewish calendar is the month of Nissan, in the spring,
when Passover occurs. However, the
Jewish New Year is in Tishri, the seventh month,
and that is when the year number is increased. This concept of different
starting points for a year is not as strange as it might seem at first glance.
The American "new year" starts in January, but the new "school year" starts in
September, and many businesses have "fiscal years" that start at various times
of the year. Similarly, the Jewish calendar has different starting points for
different purposes.

The names of the months of the Jewish calendar were adopted during the time of
Ezra, after the return from the Babylonian exile. The names are actually
Babylonian month names, brought back to Israel by the returning exiles. Note
that most of the Bible refers to months by number, not by name.

The Jewish calendar has the following months:

Hebrew

English

Number

Length

Civil Equivalent

Nissan

1

30 days

March-April

Iyar

2

29 days

April-May

Sivan

3

30 days

May-June

Tammuz

4

29 days

June-July

Av

5

30 days

July-August

Elul

6

29 days

August-September

Tishri

7

30 days

September-October

Cheshvan

8

29 or 30 days

October-November

Kislev

9

30 or 29 days

November-December

Tevet

10

29 days

December-January

Shevat

11

30 days

January-February

Adar I (leap years only)

12

30 days

February-March

Adar
(called Adar Beit in leap years)

12
(13 in leap years)

29 days

February-March

The length of Cheshvan and Kislev are determined by complex calculations
involving the time of day of the full moon of the following year's Tishri and
the day of the week that Tishri would occur in the following year. After many
years of blissful ignorance, I finally sat down and worked out the mathematics
involved, and I have added a page on The Jewish
Calendar: A Closer Look, which may be of interest to those who want a
deeper understanding or who want to write a Jewish calendar computer program.
For the rest of us, there are plenty of easily accessible computer programs
that will calculate the Jewish calendar for more than a millennium to come. I
have provided some links below.

Note that the number of days between Nissan and Tishri is always the same.
Because of this, the time from the first major festival
(Passover in Nissan) to the last major festival
(Sukkot in Tishri) is always the same.

Days of the Jewish Week

Other than Shabbat, the name of the seventh day
of the week, the Jewish calendar doesn't have names for the days of the week.
The days of the week are simply known as first day, second day, third day, etc.
Sometimes they are referred to more fully as First Day of the Sabbath, etc.
Below is a list for those who are interested.

Links to Jewish Calendars

I maintain a current Jewish calendar on this
website. Unlike most Jewish calendars you will see, my calendar shows the
Hebrew months with the corresponding civil dates.

Most printed Jewish calendars cover a 16-month period: from September of one
year (to include Rosh Hashanah) to December of the
following year. Be aware, however, that some show only the 12-month period from
September to August, and some that claim to have the full 16-month period show
only limited information about September to December of the latter year. They
show the civil months with Jewish holidays,
Torah readings, candle-lighting times and so
forth. I am particularly partial to the London Jewish Museum calendar, which
has illustrations of Jewish artwork from the middle ages to the 1800s, but
there are many Jewish calendars available on
Amazon.com.

If you would like to look up the date of a Jewish holiday, from the Gregorian
(civil) year 1 to the Gregorian year 9999, try
http://www.hebcal.com. I don't know how
accurate this is (especially given that during the earlier dates, months were
determined by observation), but I haven't caught any mistakes in it yet. Of
course, the earlier Gregorian dates are artificial, since the Gregorian
calendar did not exist until the 16th century and was not accepted in many
parts of the world until much later (they used the less accurate Julian
calendar). There is also a very nice, quick and easy converter to and from
Hebrew dates on
Chabad's
website.

If you would like to make your own computerized Jewish calendar, my page on
The Jewish Calendar: A Closer Look explains in
detail how the calendar works and provides illustrative JavaScript code.